Documentation for hook_views_data()

Patch.

Summary:
- I created an example_table table with a few different basic fields (including a nid that references node) and included the SQL to generate it in the comments.
- I then put in a lightly annotated example of views-enabling this table to try and give people guidance on where to start.

I found myself getting very turned around by information overload at http://views-help.doc.logrus.com/help/views/api-tables. What I needed was a practical example that gave me a good overview of what I'm actually doing here, and then I think the full shebang would've made a lot more sense to me.

This needs checking over for accuracy, I'm quite sure. :) Most of it was copy/paste from profile.views.inc, node.views.inc, and a few others.